🩹 [open-formulieren/open-forms#4929] Workaround react-router/dom issue

In vitest the react-router and react-router/dom imports resolve to
(CommonJS?) bundles which each contain copies of the contexts/objects
used by react router, which make you effectively end up with multiple
installations of react router. This leads to broken contexts and failing
tests, see remix-run/react-router#12785 for
more context.

Patching the resolution in vite config allows us to force loading the
mjs modules instead of the CJS build which doesn't have this problem,
because both the react-router and react-router/dom entrypoints depend
on the same chunk containing the shared code.

vite.config.mts

@@ -4,6 +4,7 @@ import {codecovVitePlugin} from '@codecov/vite-plugin';
 import replace from '@rollup/plugin-replace';
 import {sentryVitePlugin} from '@sentry/vite-plugin';
 import react from '@vitejs/plugin-react';
+import path from 'path';
 import type {OutputOptions} from 'rollup';
 import {defineConfig} from 'vite';
 import jsconfigPaths from 'vite-jsconfig-paths';
@@ -173,6 +174,16 @@ export default defineConfig(({mode}) => ({
       uploadToken: process.env.CODECOV_TOKEN,
     }),
   ],
+  resolve: {
+    alias: {
+      // ensure react-router imports don't end up with multiple copies/installations. See
+      // https://github.com/remix-run/react-router/issues/12785 for more context.
+      'react-router/dom': path.resolve(
+        './node_modules/react-router/dist/development/dom-export.mjs'
+      ),
+      'react-router': path.resolve('./node_modules/react-router/dist/development/index.mjs'),
+    },
+  },
   build: {
     target: 'modules', // the default
     assetsInlineLimit: 8 * 1024, // 8 KiB